Condenser microphone



June 22, 1965 Filed June 27,. 1962 K. SCHOEPS EI'AL CONDENSER MICROPHONE 3 Sheets-Sheet 1 INVENTO S:

'i'rfirl June 22, 1965 Filed June 27, 1962 K. SCHOEPS ETAL CONDENSER MICROPHONE S Sheets-Sheet 2 June 22, 1965 K. SCHOEPS ETAL 3,190,972

CONDENSER MICROPHONE 3 Sheets-Sheet 3 Filed June 27, 1962 IIIII FIG. 6

INVENTORS: kart Sela 1 BY Kiwi/J ni ad J: $4712 United States Patent 3,190,972 CONDENSER MICROPHONE Karl Schoeps and Willy Kiisters, Karlsruhe-Durlach, Germany, assignors to Schall-Technik, Karlsruhe-Durlach, Germany Filed June 27, 1962, Ser. No. 205,667 Claims priority, application Germany, July 8, 1961, Sch 29,963 14 Claims. (Cl. 179111) The present invention concerns condenser microphones.

In view of the development of stereophonic reproduction of sound great interest exists in condenser microphones having a plurality of alternatively selectable directional characteristics among which the 8-shape directional characteristic has assumed increasing importance.

Known condenser microphone arrangements of this type which are capable of being used with an S-shape directional characteristic have the feature in common that they are actually composed of two individual transducers which may be placed into the respective electrical circuit in various ways. In this manner transducers having different directional characteristics are combined into one arrangement. In some cases two transducers each having a unidirectional characteristic have been used in combination with each other and sometimes also in a combined structural arrangement.

One disadvantage of such microphone arrangements resides in the fact that, if they are connected in the conventional low frequency circuit with an amplifier of high resistance input necessary in this case because of the capacity of the microphone, the switching operations unavoidable With such microphone arrangements constitute a source of disturbances which may seriously affect the operation.

An even more important disadvantage of such known microphone arrangements is due to the fact that they cannot be used in connection with high-frequency cirouits although the highly developed use of transistors would greatly favor the utilization of condenser microphones in connection with high frequency circuit arrangements. The trouble is that the basic principle of alternatively varying the connection of the two individual transducers in the electrical circuit causes with every switching operation a change in the capacitance of the arrangement or produces a disturbing capacitance whereby the utilization of the microphone as one determining factor or element in an oscillatory circuit is rendered impossible. An attempt to remedy this difficulty by substitution of one of the microphones by a corresponding capacity would only result in a substantial loss of sensitivity.

It is therefore one object of this invention to provide for a condenser microphone which as a single unit permits adjustment or selection of its directional characteristics.

It is another object of this invention to provide for a microphone of the typeset forth in which also an S-shape directional characteristic is obtainable, however without the above-mentioned disadvantages of known microphone arrangements.

It is still another object of this invention to provide for a condenser microphone having the just-mentioned features and nevertheless comparatively simple in construction and entirely reliable in operation.

With above objects in view the invention includes a condenser microphone, comprising, in combination, a housing structure; a membrane mounted in said housing structure and having a front face facing the front side thereof and being adapted to constitute one condenser electrode; a counter electrode mounted in said housing structure at a predetermined spacing from the rear face of said membrane and insulated therefrom; means for conice fining a predetermined volume of air at the rear side of said membrane within said housing structure; and adjustment means for varying the directional characteristic of the microphone and including adjustable connection means for selectively controlling the connection between said volume of air and the outside air at the rear side of said housing structure which includes restricting said connection and establishing a substantially friction-free connection between said volume of air and said outside all.

The novel features which are considered a characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross-sectional elevation of one embodiment of the invention;

FIG. 2 is a plan view in direction of the arrow II of the counter electrode in the arrangement of FIG. 1;

FIG. 3 is a diagrammatic cross-sectional elevation of a second embodiment of the invention;

FIG. 4 is a similar view of a further embodiment of the invention;

FIG. 5 is a similar view of one more embodiment of the invention; and

FIG. 6 is a similar view of a further modified embodiment of the invention.

In the embodiment according to FIGS. 1 and 2 a membrane 1 is attached to a cylindrical housing 3 by means of a holding ring 2. These members may be assembled in any suitable and conventional manner e.g. by soldering, screwing or clamping. As can be seen, the membrane 1 faces one side of the housing 3 so as to be in direct contact with the surrounding air in front of the housing. Opposite to, and spaced from, the rear surface of the membrane 1 a counter electrode 4 usually called the back plate is supported in the housing 3 by means of an interconnecting insulating annular member 6. The counter electrode 4 is provided at its rear or outer side with a threaded stud 5. It will be understood that the membrane 1 made entirely or in part of electrically conductive material and the metallic back plate 4 constitute the two electrides of a condenser. Leads for connecting the two electrodes in a circuit are not shown in the drawings because they have no significance for this invention. A disc member is mounted so as to surround the stud 5 with a central bore fitting over the latter. The disc 8 has a preferably cylindrical recess 8 on its rear or outer side and a further preferably cylindrical recess 8 at its other side which faces the rear side of the counter electrode 4. The recess 8 is surrounded by a rim composed of comparatively short portions 9 projecting toward and abutting against the rear face of the counter electrode 4, openings 10 being left between consecutive projecting portions 9. A further disc member 11 with an interior thread is screwed onto the threaded portion of the stud 5 and holds the disc 8 firmly in its shown position. The counter electrode 4 is provided with a plurality of perforations 7 which are distributed over the entire area thereof in dilferent manners, e.g. in different numbers along different circles concentric with the center of the member 4 as can be seen clearly from FIG. 2. In this particular embodiment the perforations 7 in the outer marginal zone of the surface of the member 4 are more numerous and more closely spaced than the perforations 7 nearer to the center. A ring member 12 is mounted slideably in axial direction on the cylindrical circumference of the disc 8. The ring 12 is provided at its side facing the rear surface ofthe counter electrode 3 4 with a recess 13 which provides an annular air space communicating both with the marginal row or rows of perforations 7 and with the air space 16 in the recess 8 through the above-mentioned openings 10. A shallow recess 14 is provided in the margin of the disc 11 on the side thereof adjacent to the outer surface of the disc 8. The latter is provided with a plurality of perforations 15 which constitute a connection between the recess 14 and the above-mentioned air space 16. Finally, a ring member 18 is mounted slideably in axial direction on the cylindrical circumference of the disc 11 so as to be movable between the illustrated position in which the ring 18 abuts against a shoulder 20, and a second position in which the opposite face of the ring 18 abuts against the rear surface of the disc 8 in the recess 8' so as to close in this position the connection between the outer air and the recess 14.

The microphone thus described is so constructed that alternatively and selectively three different directional characteristics of the microphone can be adjusted by purely mechanical means.

In operation, when the rings 12 and 13 are in the positions shown in FIG. 1 there is only one path for sound to reach the rear side of the membrane 1 from the rear of the microphone, namely through the recess 14, perforations 15, air space 15 and the perforations 7. The recess 14 together with the opposite surface of the disc 8 constitutes a frictional element. The dimensions of the recess 14 are so chosen that it produces in cooperation with the air volume existing behind the membrane 1 a phase shift in such a manner that with this setting the microphone has a unidirectional or at least predominantly unidirectional characteristic.

If now the ring 18 is shifted in the direction of the arrow 12 into its opposite end position so as to close the access'to the recess 14, then the entire-air volume; existing behind the membrane 1 acts as an elastic stiffening cushion. It is easy to choose the shape and dimension of this air volume in designing the microphone in such a manner that the cooperation of that elastic stiffening cushion with the other elements of the oscillatory system especially the mass of the membrane complies with the well-known conditions of a pressure responsive microphone. In this case the microphone according to the invention has a spherical directional characteristic.

If the ring 12 is shifted in the direction of the arrow a from the illustrated position to a second position determined by the abutment 19, then the air volume behind the membrane 1 is in communication with the sound field behind the microphone so that in addition to the .sound impinging on the forward face of the membrane 1 also sound from the rear of the microphone can reach the rear side of the membrane 1. By design the thus established path to the rear face of the membrane is practically free of frictional effects, the microphone has now an 8-shape or substantially 8-shape directional characteristic. In this setting of the microphone it is practically immaterial whether the ring 18 is in the position illustrated in FIG. 1 or in its other end position.

While in the illustrated embodiment the frictional element formed by the annular recess 14 constitutes a predetermined constant factor, it is evidently also possible to provide for an adjustable variation of this factor. achieve this all that is needed is to set the ring 18 to positions in which the access to the recess 14 is more or less partly closed. For instance the inside of the ring 13 could be threaded and cooperate with an outside thread on the circumference of the disc 11 so that by turning the ring 18 any desired adjustment of the access to the recess 14 can be achieved.

In order to assure optimal operation of the microphone according to the invention it is advisable to provide the counter electrode 4, as mentioned above, in its marginal zone cooperating with the ring 12 with a comparatively large number of perforations. The same effect may be obtained by giving the individual perforations in the marginal zone comparatively large cross-section. As a rule it will be advisable to arrange matters in such a manner that the sum of the cross-sections of the perforations in said marginal zone is at least equal to, or preferably larger than, the sum of the cross-sections of the perforations in the remaining inner area of the counter electrode 4. Such an arrangement is illustrated by way of example in FIG. 2.

In FIG. 3 which illustrates another embodiment of the invention all those elements which are identical with those shown in FIGS. 1 and 2 are designated by the same reference numerals. In this embodiment a disc 21 is mounted behind the counter electrode 4 and is held in assembled position by being threaded onto a threaded portion of the stud 5. Thedisc 21 is provided on the side facing the rear face of the member 4 with a recess 22 surounded by a continuous rim 2.3 abutting against the insulating ring 6 and fitting the circumference of the member 4. The ring 21 is provided with large openings 24. The outer smooth end of the stud 5 carrie a disc 25 which is slideable in axial direction along the portion 5 of the stud 5. In the illustrated position of the disc 25 a portion of its inner face abuts against the outer face of the disc 21. The other outer end position of the disc 25 is determined by a snap ring 26. The inner face of the disc 25 is provided with a shallow recess 27 which establishes in this position a narrow access for sound from the outside air in the rear of the microphone to the openings 24.

It will be understood that the disc 25 serves to establish in the illustrated position a friction through the narrow path across recess 27 left between the opposing faces of the disc 25 and the disc 21. If, however, the disc 25 is moved in the direction of the arrow 0 into its other end position then the surrounding air behind the microphone communicates freely through the openings 24 with the air volume otherwise existing behind the membrane 1 so that in this setting of the microphone an 8-shape or predominantly S-shape directional characteristic of the microphone is obtained. In this embodiment only two alternative settings of the microphone are provided.

It should be noted that in the embodiment according to FIG. 3 a sound-permeable plate 28 is mounted on top of the holding ring 2. This plate may be perforated or may be may for example of metal mesh or fabric or any other suitable porous material. An air volume 29 exists between the plate 28 and the membrance 1. By properly selecting the material, its arrangement and its dimensions it is possible to provide for a phase shift of the sound waves impinging on the front face of the membrane whereby if necessary deviations of the directional characteristic from a desired form thereof may be compensated.

The embodiment according to FIG. 3 may be slightly modified by not providing the recess 27 in the disc 25. In this case its rear face would abut entirely in the shown position of disc 25 against the outer face of the disc 21 and completely close the openings 24. Hereby the air volume existing behind the membrane 1 would be completely closed off against the outside air. Assuming that the air volume behind the membrane is properly dimensioned, then a microphone is obtained which has, in one position of the disc 25, not a unidirectional characteristic but a spherical characteristic, and in the other position of the disc 25 an S-shape directional characteristic.

The embodiment according to FIG. 3 is based on the concept that a disc 25 is'shifted to an end position in which a friction-free connection between the outside air in the rear of the microphone with the air volume existing behind the membrane is established whereby the microphone is set for an 8-shape directional characteristic. It was further understood that the disc 25 is either provided with a recess 27 for establishing a frictional element or is formed without this recess 27 so as to be able to completely close in the illustrated position the openings 24.

In contrast therewith FIG. 4 illustrates a modification in which a member corresponding to the disc 25 is stationarily mounted on the stud 5 while a separate perforated disc is shiftable between a position in which free access of the outside air to the membrane or a frictional element can be established. As can be seen in FIG. 4, a disc 31 is screwed onto the threaded portion of the stud 5 and held in that position for instance by a nut 5". A disc 32 having small perforations 37 at a certain radial distance from the center and larger perforations 33 at a lesser distance from the center is shiftable in axial direction between a position, as illustrated, in which its outer face abuts against the inner face of the disc 31, and a position in which the slightly projecting inner rim of the disc 32 abuts against the outer face of the counter electrode 4. The variable air volume 34 between the counter electrode 4 and the disc 32 is confined by a projecting rim 35 of the counter electrode 4 which fits closely over the circumference of the disc 32. It can be seen that when the disc 32 is shifted in the direction of the arrow d to the above-mentioned second position thereof, the air volume behind the membranes 1 is in practically friction-free connection through the large openings 33 with the outside air behind the microphone. In this case the microphone has the desired 8-shape directional characteristic. If the latter is to be changed to a unidirectional characteristic then the disc 31 is to be provided with a shallow recess 36 cooperating with the small perforations 37 so that in the illustrated position of disc 32 only a predetermined narrow path for sound waves from the rear of the microphone to the inner air volume behind the membrane 1 is provided. However, the separate narrow perforations 37 may also be omitted in which case the shallow recess 36 has to extend so much toward the center of the microphone that, in the same manner as in FIG. 3, the recess 36 communicates with the openings 33.

FIG. 5 illustrates a further embodiment in which the characteristic of the microphone can be changed between unidirectional characteristic and 8-shape directional characteristic. The member 41 designed to constitute the rearward boundary of the air volume existing behind the membrane 1 is held in the illustrated position on the stud 5 by a disc 42 screwed onto the threaded portion of stud 5 whereby the outer rim of member 41 is caused to abut against the insulating ring 6 while also fitting closely over the circumference of the counter electrode 4. In a manner analogous with that illustrated in the preceding examples the disc 42 is provided with a shallow recess 43 communicating with correspondingly positioned perforations 41' in the member 41. In this manner a friction element for producing a unidirectional characteristic is provided for. The member 41 is provided in its marginal area with large perforations 44 communicating with the air volume behind the membrane 1 and closed against the outside air in the rear of the microphone by a ring member 46 when the latter is in the illustrated position. However, the ring member 46 is slideable in axial direction along the circumference of the annular portion 45 of member 41. If the ring 46 is moved from the illustrated position in the direction of the arrow 2 to end position determined by the shoulder 45, then a practically friction free connection of the outside air behind the microphone through openings 44 with the inner air volume behind the membrane 1 is established. Hereby the desired 8-shape directional characteristic is obtained.

FIG. 5 illustrates a further modification of the arrangement according to the invention so as to show by way of example that this microphone may also be operated in push-pull fashion. For this purpose a second counter electrode 48 with perforations 49 is mounted opposite the front face of the membrane 1 and is supported by an insulating ring 47 arranged on the holding ring 2. It will be understood that this second counter electrode 48 may also be formed in such a manner that it may simultaneously serve as an element producing a phase shift of the sound waves impinging on the front face of the membrane 1.

In all the above-described and illustrated embodiments it may be assumed that the movable elements are moved either by hand or by mechanical means not shown. However, it may be sometimes desirable to adjust the microphones and to move the movable elements by remote control. For this purpose for instance the members 12 and 18 of FIG. 1 or the movable elements of the other embodiments may be made of ferromagnetic material and may be moved into one of their positions by the application of a magnetic field against the action of an elastic returning force or element so that they are returned to their other position upon the disappearance of the magnetic field.

FIG. 6 illustrates by way of example a microphone according to the invention which can be operated with remote control. Generally the structure of the microphone according to FIG. 6 is analogous to that illustrated by FIG. 1. Additionally solenoid coils 51 and 52 are provided and so arranged that upon energization thereof the ferromagnetic members 12 and 18, respectively, are moved in the direction of the arrows 7 and g, respectively, against the elastic resistance of the members 53 and 54, respectively, e.g. rubber rings. The dimensionsof the cooperating parts are so chosen that the elastic force of the members 53 and '54 is sufficient for moving the members 12 and 18, respectively, safely into the closing positions illustrated in FIG. 6, and that the members 53 and 54 yield sufficiently to permit the solenoids 5'1 and 52, respectively, to move the members 12 and 18, respectively, sufficiently to their opening positions. It is evident that by individual energization of the solenoids 51 and 52 the members 12 and 18 can be operated individually and independently from each other. Consequently also the microphone according to FIG. 6 can be adjusted selectively to assume either one of the above-mentioned three different directional characteristics.

Of course, in the case of stereophonic sound recording or transmission tw-o microphones according to the invention may be combined with each other in a manner generally known per se in which case the two microphones are preferably set for reception in two different directions and are arranged closely adjacent to each other and preferably one above the other in order to determine in this way a more or less common reference point.

It will be understood that each of the elements described above, or two or more together, may also fin-d a useful application in other types of a condenser microphone differing from the types described above.

While the invention has been illustrated and described as embodied in a condenser microphone adjustable between a plurality of directional characteristics, it is not intended to be limited to the details shown, since various modifications and .structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully revealt-he gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A condenser microphone, comprising, in combination,

a housing structure;

a condenser electrode including an at least partly conductive membrane mounted in said housing structure and having a front face facing the front side thereof;

a counter electrode mounted in said housing structure 7 at a predetermined spacing from the rear face of said membrane; 7

means for insulating said electrodes from each other;

means for confining a predetermined volume of air at the rear side of said membrane within said housing I structure;

first air passage means for establishing a restricted connection between said volume of .air and the outside air at the rear of said'housing structure;

second air passage means for freely connecting said volume of air and said outside air with each other; and

mechanical adjustment means for varying the directional characteristic of the microphone and including movable control means which in a first position render said second air passage means inoperative and leave said first air passage means operative only to a predetermined degree so that the microphone obtains a directional characteristic depending upon said degree of operativeness of said first air passage means and which in a second position render said second air passage means operative so that irrespective of the operative condition of said first air passage means a 'bi-directional 8-shape characteristic of the microphone is established by the friction-free connection between said volume of air and said outside air.

2. A microphone as claimed in claim 1, wherein said housing structure includes a stationary closure member along its rear side and defining an air space between itself and the rear face of said counter electrode, said counter electrode and said closure member being provided each with a plurality of perforations for establishing a connection from the air immediately behind said membrane and from said air space to said outer air, and wherein said adiustment means include control means movable between a first position in which said volume of air is in substantially friction-free connection with said outside air through at least some of said perforations, and a second position in which said connection is restricted to a predetermined degree.

3. A microphone as claimed in claim 2, wherein said counter electrode is provided with a first group of perforations in its marginal zone and a second group of perforations in the area surrounded by said marginal zone, the sum of the cross-sectional areas of said first group of perforations being at least equal to the sum of the crosssectional areas of said second group of perforations, said control means comprising a first control member movable between a first position in which said perforations are in free communication with said outside air, and a second position in which said perforations are closed against said outside air, and wherein said control means comprise a second control member movable between a first position in which said perforations of said closure member are in frictional communication with said outside air, and a second position in which said last mentioned perforations are closed against said outside air.

4. A microphone as claimed in claim 3, wherein said housing structure includes a stationary member mounted on the outside of said closure member and including a narrow channel portion communicating with said perforations of said closure member and with said outside air and located so as to constitute a frictional connection between said volume of air and said outside air when said second control member is in said first position thereof, and to be closed when said second control member is in said second position thereof.

5. A microphone as claimed in claim 2, which said control means include a control member provided with a narrow channel portion communicating with said perforations of said closure member and with said outside air, said control member being movable between a first posi tion in which said perforations of said closure member are in friction-free communication with said outside air, and a second position in which said perforations are in communication with said outside air only through said channel portion so as to establish a frictional connection between said volurne of air and said outside air.

6. A microphone as claimed in claim 2 wherein said closure member is provided with a first group of perforations in one area and a second group of perforations in a second area thereof, narrow channel means being arranged in communication with said second group of perforations of said closure member and with said outside air for establishing a frictional connection between said volume of air and said outside air, and wherein a control member is movable between a first position in which said first group of perforations of said closure member is in friction-free communication with said outside air, and a second position in which said first group of perforations is closed against said outside air.

7. A microphone as claimed in claim 1, wherein said housing structure comprises a stationary member arranged to leave a space between itself and the rear face of said counter electrode and provided with narrow channel means open to said space and to said outside air, said counter electrode being provided with a plurality of perforations, and wherein a control member provided with a plurality of perforations is movable in the space between said rear face of said counter electrode and said stationary member and between a first position in which at least some of said perforations of said control member are in friction-free communication with said outside air, and a second position in which at least some of said perforations of said control member are in communication with said outside air only through said narrow channel means so as to establish a frictional connection between said volume of air and said outside air. 7 V

S. A microphone as claimed in claim 1, including phase shift means mounted opposite said membrane on the front side of said housing structure for influencing the sound waves impinging on said front face of said membrane.

,9. A microphone as claimed in claim 1, including a second counter electrode mounted on said housing structure at a predetermined spacing from the front face of said membrane and insulating means for insulating it therefrom, for rendering the microphone operable in pushpull fashion.

it A microphone as claimed in claim 1, including means for operating said adjustment means by remote control.

11. A microphone as claimed in claim 10, wherein said adjustment means include control means movable between two positions and made at least partly of ferromagnetic material, solenoid means energizable for moving 7 said control means from one position in one direction to a second position, and return means for returning said control means from said second to said one position when said solenoid means are de-energized.

12. A condenser microphone, comprising, in combination,

a housing structure;

a condenser electrode including an at least partly conductive membrane mounted in said housing structure and having a front face facing the front side thereof;

a counter electrode mounted in said housing structure at a predetermined spacing from the rear face of said membrane;

means for insulating said electrodes from each other;

means for-confining a predetermined volume of air at the rear side of saidmembrane within said housing structure, said predetermined volume of air being in relation to the mass of said membrane so dimensioned that the microphone has a predetermined polydirectionalcharacteristic when connection between said volume of air and the air surrounding the microphone is blocked;

first air passage means for establishing a restricted connection between said volume of air and the outside air at the rear of said housing structure;

second air passage means for freely connecting said volume of air and said outside air with each other; and

mechanical adjustment means for varying the directional characteristic of the microphone and including movable control means which in a first position render said second air passage means inoperative and blocks said first air passage means so that the microphone obtains said poly-directional characteristic, and which in a second position render said second air passage means operative so that irrespective of the operative condition of said first air passage means a bi-directional 8-shape characteristic of the microphone is established by the friction-free connection between said volume of air and said outside air.

13. A condenser microphone, comprising, in combination,

a housing structure;

a condenser electrode including an at least partially conductive membrane mounted in said housing structure and having a front face facing the front side thereof;

a counter electrode mounted in said housing structure at a predetermined spacing from the rear face of said membrane;

means for insulating said electrodes from each other;

means for confining a predetermined volume of air at the rear side of said membrane within said housing structure, said predetermined volume of air being in relation to the mass of said membrane so dimensioned that the microphone has a predetermined polydirectional characteristic when connection between said volume of air and the air surrounding the microphone is blocked;

first air passage means for establishing a restricted connection between said volume of air and the outside air at the rear of said housing structure;

second air passage means for freely connecting said volume of air and said outside air with each other; and

-mechanical adjustment means for varying the directional characteristic of the microphone and including movable control means which in a first position render said second air passage means inoperative and blocks said first air passage means so that the microphone obtains said poly-directional characteristic, which in a second position render said second air passage means inoperative and leave said first air passage means operative to a degree so restricted as to constitute a frictional connection between said volume of air and said outside air whereby the microphone ob- 5Q is established by the friction-free connection between said volume of air and said outside air.

14. A condenser microphone, comprising, in combination,

a housing structure;

a condenser electrode including an at least partly conductive membrane mounted in said housing structure and having a front face facing the front side thereof;

a counter electrode mounted in said housing structure at a predetermined spacing from the rear face of said membrane;

means for insulating said electrodes from each other;

means for confining a predetermined volume of air at the rear side of said membrane within said housing structure, said predetermined volume of air being in relation to the mass of said membrane so dimensioned that the microphone has a predetermined polydirectional characteristic when connection between said volume of air and the air surrounding the microphone is blocked;

first air passage means for establishing a restricted connection between said volume of air and the outside air at the rear of said housing structure;

second air passage means for freely connecting said volume of air and said outside air with each other; and

mechanical adjustment means for varying the directional characteristic of the microphone and including movable control means which in a first position render said second air passage means inoperative and leave said first air passage means operative only to a predetermined degree so restricted as to constitute a frictional connection between said volume of air and said outside air whereby the microphone obtains a substantially unidirectional characteristic, and which in a second position render said second air passage means operative so that irrespective of the operative condition of said first air passage means a bi-directional S-shape characteristic of the microphone is established by the friction-free connection between said volume of air and said outside air.

References Cited by the Examiner UNITED STATES PATENTS 2,387,845 10/45 Harry 179-111 2,852,629 9/58 Schoeps et a1 l791ll 2,939,922 6/60 Gorilce 179-1 2,958,739 11/60 Schoeps et al 179111 OTHER REFERENCES The Electrostatic Earphone, by Walter T. Selsted, Journal of the Audio Engineering Society, April, 1961, vol. 19, No. 2, pp. -147.

ROBERT H. ROSE, Primary Examiner. 

1. A CONDENSER MICROPHONE, COMPRISING, IN COMBINATION, A HOUSING STRUCTURE; A CONDENSER ELECTRODE INCLUDING AN AT LEAST PARTLY CONDUCTIVE MEMBRANE MOUNTED IN SAID HOUSING STRUCTURE AND HAVING A FRONT FACE FACING THE FRONT SIDE THEREOF; A COUNTER ELECTRODE MOUNTED IN SAID HOUSING STRUCTURE AT A PREDETERMINED SPACING FROM THE REAR FACE OF SAID MEMBRANE; MEANS FOR INSULATING SAID ELECTRODES FROM EACH OTHER; MEANS FOR CONFINING A PREDETERMINED VOLUME OF AIR AT THE REAR SIDE OF SAID MEMBRANE WITHIN SAID HOUSING STRUCTURE; FIRST AIR PASSAGE MEANS FOR ESTABLISHING A RESTRICTED CONNECTION BETWEEN SAID VOLUME OF AIR AND THE OUTSIDE AIR AT THE REAR OF SAID HOUSING STRUCTURE; SECOND AIR PASSAGE MEANS FOR FREELY CONNECTING SAID VOLUME OF AIR AND SAID OUTSIDE AIR WITH EACH OTHER; AND MECHANICAL ADJUSTMENT MEANS FOR VARYING THE DIRECTIONAL CHARACTERISTIC OF THE MICROPHONE AND INCLUDING MOVABLE CONTROL MEANS WHICH IN A FIRST POSITION RENDER SAID SECOND AIR PASSAGE MEANS INOPERATIVE AND LEAVE SAID FIRST AIR PASSAGE MEANS OPERATIVE ONLY TO A PREDETERMINED DEGREE SO THAT THE MICROPHONE OBTAINS A DIRECTIONAL CHARACTERISTIC DEPENDING UPON SAID DEGREE OF OPERATIVENESS OF SAID FIRST AIR PASSAGE MEANS AND WHICH IN A SECOND POSITION RENDER SAID SECOND AIR PASSAGE MEANS OPERATIVE SO THAT IRRESPECTIVE OF THE OPERATIVE CONDITION OF SAID FIRST AIR PASSAGE MEANS A BI-DIRECTIONAL 8-SHAPE CHARACTERISTIC OF THE MICROPHONE IS ESTABLISHED BY THE FRICTION-FREE CONNECTION BETWEEN SAID VOLUME OF AIR AND SAID OUTSIDE AIR. 